Various types of power or amplitude adjustment circuits may be employed in a system. The terms “amplitude” and “power” are generally used interchangeably herein, and so references to power are intended to likewise encompass amplitude. In many radio frequency (RF) circuits, for example, power adjustment circuitry, such as amplifiers and/or attenuators are often employed. The term “gain” generally refers to a measure of the ability of a power adjustment circuit to increase the power of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a power adjustment circuit to the signal input of the same circuit. It may also (or alternatively) be defined on a logarithmic scale, in terms of the decimal logarithm of the same ratio (“dB gain”).
Generally, an amplifier (or “amp”) refers to any device that changes (e.g., usually increases) the power of a signal. The relationship of the input to the output of an amplifier, usually expressed as a function of the input frequency, is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain. In audio applications, amplifiers drive the speakers used in public address (PA) systems to make the audio content that is output louder, for example.
An attenuator generally refers to any electronic device that changes (e.g., usually reduces) the power of a signal without appreciably distorting its waveform. Variable attenuators may dynamically change the extent to which they alter (e.g., reduce) the power of a signal. Thus, by adjusting the attenuation to increase loss, an attenuator may effectively reduce the power (or gain) of a signal. On the other hand, by adjusting the attenuation to decrease the amount of loss, the attenuator may effectively increase its gain from one point in time to a next point in time. For instance, by reducing the amount of loss from a first point in time to a second point in time, the attenuator may be viewed as effectively increasing the power from that observed at the first point in time to that observed at the second point in time, even though the overall effect by the attenuator on a received input signal in both instances might be to reduce the power thereof in its output signal.
Accordingly, amplifiers and attenuators are two types of power adjustment circuits that may be employed for altering power (or gain) of a signal. In many applications a series of power adjustment stages (which may be referred to as a series of “gain” stages) are implemented, wherein the power of a signal propagating through the stages may be adjusted at any one or more of the stages. For instance, a series of power adjustment stages may each include an attenuator (or amplifier) for adjusting the power (or gain) of a signal. The series of power adjustment stages may be implemented, for example, to allow for finer control over adjustments made to the power of a signal, as is well-known in the art. As used herein, each power adjustment stage may be referred to as a “gain” stage, even though attenuators may be employed at some stages for potentially decreasing, rather than increasing, the power of an input signal. Accordingly, a “gain” stage is not limited to stages that increase the power of a signal received into the stage.
The control of a series of gain stages may pose several challenges, however, due in part to limited attenuation ranges which can cause downstream irreversible inter-modulation or blocking. When an attenuator runs out of gain or attenuation, the actions of upstream attenuators can create distortion or noise conditions that cannot be reversed in downstream attenuators. Because the power level being controlled at every point in the series of stages is the cumulative result of all prior (upstream) stages, the actions of upstream stages (or loops) can interfere with the ability of downstream stages (or loops) to control their local power level. The series of stages then become coupled and a bottom-up approach can lead to undesired behavior such as oscillations and non-optimum signal-to-noise and distortion (SINAD).
Automatic gain control (AGC) refers generally to an adaptive system found in many systems or electronic devices. In general, an AGC may autonomously (without requiring human input) control the settings/parameters of a power adjustment circuit for managing the amount of gain that it imparts to a signal. Typically, the average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels. For example, without AGC the sound emitted from an AM radio receiver would vary to an extreme extent from a weak to a strong signal; the AGC effectively reduces the volume if the signal is strong and raises it when it is weaker. AGC algorithms often use a proportional-integral-derivative (PID) controller where the P term is driven by the error between expected and actual output amplitude.
In conventional systems that employ a series of gain stages, each stage includes a localized control loop that is specific to that stage's respective power adjustment circuit (e.g., attenuator), and such localized control loop acts only on information that is present locally (in its respective stage). However, the inventor(s) of the present application have recognized that such localized control loops for each stage may lead to problems or oscillations where one localized control loop desires to take some action that may have a negative impact on another downstream stage. Accordingly, because each gain stage conventionally controls itself individually based on its localized data, the overall impact on the series of gain stages may not be controlled or managed very well.
Consider, for example, a series of three (3) gain stages that each includes an attenuator. The attenuator of the first gain stage receives an input signal and produces a first attenuated output signal. The attenuator of the second gain stage then receives as input the first attenuated signal output by the first gain stage and produces a second attenuated output signal. Finally, the attenuator of the third gain stage receives as input the second attenuated signal output by the second gain stage and produces a third attenuated output signal. In general, each stage may have a localized control loop to try to produce an output signal that best addresses noise and distortion. For instance, each stage may attempt to keep the power high enough to reduce noise problems, while keeping the power low enough to minimize distortion.
Suppose now that, in the above example of the 3-stage series, the second attenuator observes that it has the range to increase the power of its output signal, which may be desirable from a pure localized view of that gain stage (e.g., to minimize its noise). However, further suppose that the attenuator in the third gain stage is set to its maximum attenuation such that it is unable to further reduce its output power. In this instance, the increased power output by the second stage may cause the third stage to incur distortion which the attenuator of such third stage is unable to address (because it is already set to its maximum attenuation).
In certain systems, there may be some communication channel that allows the third gain stage to notify the second gain stage that its output power is too high and is causing distortion that the third gain stage is unable to fix, and thereby request the second gain stage to reduce its output power. This type of situation, however, is the very definition of an oscillation. In other words, an upstream gain stage made a change that it did not predict could cause a problem in a downstream stage because the upstream gain stage decided to make the change based only on its local information. Because the change made upstream has to later be reversed in order to correct for the problem caused downstream, it results in an oscillation, rather than avoiding the upstream change in the first place.